Unconfined, Semi-confined and Confined Aquifers - National Water Commission

Remember... the Wappentake Creek is not ephemeral...and "the basement aquifer is at least partially confined"... let's see what that last bit - "at least partially confined" - actually means; besides, of course "at least partially unconfined"...

From Guidance for groundwater storage utilisation in water planning - Water Report Series No. 81, June 2012:
(A series of works commissioned by the National Water Commission on key water issues)

From section 2.3 Unconfined, semi-confined and confined aquifers (p. 6):

Whilst there is a significant distinction between completely confined and unconfined aquifers, there is a continuum of properties between them that is the domain of the more common ‘semi-confined’ (or ‘leaky’) aquifers. These are neither completely confined nor unconfined, but are generally fully saturated, and overlain by ‘leaky’ strata through which recharge and discharge can occur illustrates potential relationships between common aquifer types (SKM 2011) and the unconfined, semi-confined and confined classification.

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From section 4.3.4 Setting quantitative objectives and targets:

environmental objectives for groundwater systems are likely to include: overall long-term system extraction limits to ensure water levels/pressures do not continue to decline (i.e. that there is no long-term ‘mining’ of the resource); water level/pressure thresholds that must be maintained (at all times or from time to time) to protect base flows and other dependent ecosystems (these will be particularly important in unconfined groundwater systems); possibly maximum or other rates of water level/pressure change to protect GDEs; and water quality objectives and associated water level targets that ensure the beneficial use of the resource is not compromised (this is also important for supply sustainability).

interception. Allowing utilisation of groundwater storage will result in greater groundwater extraction. In most confined systems, this water will come largely from elastic storage. Recharge from adjacent aquifers and/or surface water systems will occur, but is unlikely to be increased appreciably. Discharge will be reduced; however, in many of these deep confined systems this discharge is to more saline systems, which themselves may take hundreds, if not thousands, of years to reach surface water bodies as saline water. In many unconfined systems, however, taking from storage will result in both induced recharge from surface systems, and reduced discharge to adjacent groundwater systems and surface systems. This ‘interception’ may have impacts not only on environmental values, but also on the rights of existing water entitlement holders in those connected surface water or groundwater systems. This is particularly an issue for connected systems that are already fully committed, and even more so in the Murray-Darling Basin, where surface water use is capped.

In reality, most ‘confined’ systems are actually semi-confined, and to varying degrees are connected to aquifers above them. Changes in deeper aquifers will often manifest as declines in overlying unconfined aquifers, sometimes quickly and sometimes over several years. In such cases, the impact on objectives of overlying systems, and of surface systems connected to these, also must be taken into account.

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From section 4.3.5 Developing groundwater management strategies and undertaking risk assessment (p. 40):

 Social risk assessment considerations might include:

·        the social impacts of the proposed options and alternatives for groundwater storage utilisation

·        the equity issues associated with the groundwater storage utilisation options and alternatives

·        the adjustment costs to satisfy the equity principles of reform.

Social and cultural values relating to water are often in conflict with economic values. For many people, water has special cultural and social values, and should not be treated solely as an economic good. That is, economic efficiency should not always be the prime determinant of water allocation. Utilisation of groundwater in storage can create greater groundwater level drawdown or pressure declines than would otherwise occur. This can create social impacts by limiting the access to groundwater by some users (e.g. for stock and domestic users), and compromising the water rights in connected systems or the enjoyment community groups derive from groundwater dependent ecosystems, including base flows in connected surface water systems.

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Published by the National Water Commission
95 Northbourne Avenue
Canberra ACT 2600
Tel: 02 6102 6000
Email: enquiries@nwc.gov.au
Date of publication: May 2012